What do mammals have to say about the neurobiology of acoustic communication?

Auditory communication is crucial across taxa, including humans, because it enables individuals to convey information about threats, food sources, mating opportunities, and other social cues necessary for survival. Comparative approaches to auditory communication will help bridge gaps across taxa and facilitate our understanding of the neural mechanisms underlying this complex task. In this work, we briefly review the field of auditory communication processing and the classical champion animal, the songbird. In addition, we discuss other mammalian species that are advancing the field. In particular, we emphasize mice and bats, highlighting the characteristics that may inform how we think about communication processing.


Introduction
The field of neuroethology heavily relies on the natural characteristics that make different species well-suited to study particular questions.Within the field, we lean on Krogh's principle that "For such a large number of problems, there will be some animal of choice or a few such animals on which it can be most conveniently studied"; now we can move a step forward and expand the champion animals for the field of auditory communication to take advantage of a comparative perspective.Other approaches focus on implementing state-of-the-art techniques on a few genetically tractable animal models, such as mice, flies, and worms.Today, we can benefit from a different mindset: new insights emerge by observing innate behaviors in classical species and applying innovative techniques, melding neuroethology and systems neuroscience.Here, we briefly review the classical animal model for the study of acoustic communication, songbirds.We will highlight the significant and fundamental groundwork this model system provided in animal communication and then advocate for how the influential discoveries serve as a springboard for studying related questions in different mammals.Broadly speaking, we will first focus on a wide range of mammals but ultimately underscore the advantages, limitations, and untapped opportunities for bats and mice.

The backbone of the field: Songbirds
Songbirds have been the long-standing model organism for vocal learning and acoustic communication 1 .The acoustic and syntactic complexity of their songs and the ability of many avian species to learn these sounds provide exceptional correlates to human speech 2 .Song complexity has evolved under sexual selection, as most females choose mates according to song quality 3 .This evolution of song complexity produced a treasure trove of vocal repertoires exploited to study auditory signal production and processing simultaneously.For example, fundamental studies showed that maintaining a stable song depends on proper auditory feedback in several songbird species 4 .Also, many studies have described song-selective neurons in the caudomedial nidopallium and Field L, the avian homolog of the auditory cortex 5 .Neurons tuned to species-specific sounds are found in these brain regions, and the responses of these neurons depend on social context 6,7 .
These findings suggest that neural selectivity to species-specific sounds may facilitate vocal learning through auditory feedback 8 Selectivity for conspecific calls is also evidenced by a strong response bias to natural calls instead of synthetic signals 9 .Additional studies revealed that neuronal selectivity in the caudomedial nidopallium and Field L is driven by discrete frequency components and spectral contrast of vocalizations-not by harmonicity 10 .These findings shed light on the acoustic features that may carry behavioral information.Moreover, salient acoustic features such as temporal and spectral modulations drive more robust neural responses than characteristics such as frequency modulations 11 .Though much work has focused on how acoustic features drive neuronal selectivity, it is less clear how and what specific acoustic features of natural vocalizations drive neuronal selectivity rather than synthetic stimuli.In a secondary auditory area, the medial caudal mesopallium, some neurons are tuned to specific acoustic features (motifs), and this selectivity depends on the birds' prior experience 12 .Familiarity with vocalizations appears to be critical for selectivity in most auditory processing areas of songbird brains 13 , highlighting the importance of top-down processing in sound classification.
Birds are specialists in vocal communication and exhibit extensive repertoires of learned songs.These insights have helped start unmasking the neural mechanisms of social communication.However, limiting our focus to songbirds may constrain our understanding of the neural basis of social communication, as there are differences between avian and mammalian brains.Moreover, it is essential to bridge experimental and analytical designs to extract fundamental principles across taxa 14 .
Here and now: Mammalian models Acoustic communication is commonly observed across a wide range of mammals.Each species provides a robust system for elucidating various facets of how information is transferred between animals and the neural networks that underlie this sophisticated process.In humans, auditory processing of communication has focused on psychophysical studies of language perception, EEG, fMRI, and the underlying genetics of language disorders.While significant advances in our understanding of how human brains process language transpired over the last decades, the approaches have many limitations in revealing circuit mechanisms and rapid neural dynamics underlying communication processing.Thus, animal models remain a necessity in the exploration of these topics.Delphinids, a family of approximately 35 species of dolphins, have a complex, adaptable vocal repertoire and show vocal learning 15 .Moreover, the closest living terrestrial animal to these aquatic mammals is the hippopotamus 16 , thus making delphinids an ideal model for studying the evolution of vocal communication.Marmosets (Callithrix jacchus), New World primates, are emerging as a prominent model for studying vocal communication 17 .These primates have a rich vocal repertoire consisting of simple and compound calls 18 that appear to be elicited or suppressed by vocal-only neurons in the premotor cortex 19 .Prairie voles (Microtus ochrogaster), small rodents found in North America, are a powerful model for uncovering the link between hormones, neural circuits, and social behavior.Groundbreaking work by Amadei et al. 20 revealed that corticostriatal activity enhances female prairie vole huddling, an affiliative behavior believed to be regulated by oxytocin and other hormones 21,22 .Because adult prairie voles vocalize during these social interactions 23 the possibility exists for using this mammalian model to study vocal communication in conjunction with hormones, brains, and naturalistic behavior.Naked mole rats (Heterocephalus glaber) are burrowing rodents native to Africa.While in physical contact with other conspecifics, these mammals utilize antiphonal calling 24 .The vocal repertoire of Heterocephalus glaber consists of at least 17 distinct vocalizations 25 .One category, the soft chirp, transmits information about colony membership and can be learned by pups cross-fostered in a foreign colony, thus opening the possibility of exploring whether vocal learning exists in this mammalian species 26 .Alston's singing mice (Scotinomys teguina) are vocally interactive neotropical rodents.These vocal interactions are cortically dependent, temporally precise, and socially modulated 27 .Both males and females participate in vocal interactions consisting of discrete frequency-modulated harmonic broadband notes 28,29 .Because of the tight precision between calling and responding, Alston's singing mice are an ideal model for understanding the temporal dynamics linking auditory processing and motor control.
Each of these species provides a unique advantage in studying communication processing, and a comparative approach might profoundly advance our understanding of the neural basis of social communication.In particular, two taxa of mammals in which significant progress has been made in recent years are bats and mice, and we delve into this research in the following sections.

Mice as mammalian models for auditory communication
Like other mammalian species, mice utilize a diverse acoustic repertoire signaling with both audible, low-frequency squeaks 30 and ultrasonic, high-frequency vocalizations 31 .Lowfrequency vocalizations are typically associated with mating, pain, and fear 32,33 , but the sound's meaning appears context-dependent.On the other hand, the purpose of mouse ultrasonic vocalizations (USVs) is an ongoing debate.The information conveyed by these signals, however, might be context-dependent.For example, when young male and female pups fall out of the nest, they emit USVs, and then dams retrieve them 34,35 .As juveniles, mice stop vocalizing, although why this vocal hiatus exists is still unclear 36 .Adult male mice produce ultrasonic vocalizations when singly housed and exposed to female urine 37 or other scent cues 38 .Although rarer, female mice also vocalize when exposed to male urine 39 .As adult animals socialize, vocal production is prevalent when a female cohabitates with another female 40,41 .In some strains, pairing males elicits vocal production 42,43 .
During courtship, a commonly held assumption is that only male mice vocalize [44][45][46] .With the advent of new technology, that enables sound source separation and localization, experimenters could determine where a USV originated.This technological advancement allowed multiple groups to reveal that both sexes vocalize during courtship [47][48][49] .Mouse vocalizations also hold translational value.For instance, impairments in communication are characteristic of multiple neurological disorders.Groszer et al. 50showed that mice with a point mutation in FOXP2, a gene associated with human speech deficits 51 , emit complex innate USVs have deficits in motor skill learning, and display synaptic plasticity impairments.Chabout and colleagues 52 extended this finding by showing that FOXP2mutant mice produce USVs with a different temporal patterning than controls.Similarly, mouse models of autism show deviations in the acoustic features of vocalizations and emission rate compared to controls [53][54][55] .However, it remains an open question on how these altered patterns in vocal signaling shape behavior and what information these auditory cues convey.
Progress toward decoding the meaning of different ultrasonic vocalizations has advanced significantly after multiple groups started using molecular tools that allow precise control over specific neurons and identification of vocalizing animals.Sangiamo et al. 56 demonstrated that specific ultrasonic vocal signals are associated with distant social behaviors.These signals also alter the behavior of a socially engaged partner, highlighting the communicative role mouse USVs play in social behavior.Work by Chen et al. 57 and Tschida et al. 58 unmasked the neural circuitry underlying USV production.Chen and colleagues showed that activating a distinct class of lateral preoptic area (LPOA) neurons elicited vocal production that resembled the repertoire of USVs produced by control animals.These LPOA neurons express oestrogen receptor 1 and project to the periaqueductal grey (PAG).Related, Tschida and colleagues demonstrated that activating PAG neurons tagged during USV emission gate affected downstream vocal-patterning circuits.These seminal discoveries depended on innovative genetic approaches to identify and control neurons in mice.While these experiments have illuminated the necessity of midbrain and hindbrain circuity in vocal production, many unresolved questions remain.For example, a long-standing theory of acoustic communication suggests that an animal's motivational state underlies the classes of vocalizations produced 59 .Indirect evidence supports this hypothesis, but how motivation affects the circuitry controlling vocal production is unclear.By taking advantage of the tools optimized to probe the neural circuitry of mice, elucidating this mystery and many others in mammalian communication is possible.

Bats and their contributions to understanding acoustic communication
Echolocating bats navigate in darkness by producing ultrasonic vocalizations and listening to the echoes generated by objects in their environment.Bats extract differences in echo intensity, spectrum, and binaural timing comparisons to determine the time delay between sonar emission and echo return.These timing differences allow bats to account for distance, thus accurately computing the position of prey and other objects in the environment [60][61][62] .Bats are audio-vocal specialists that can adapt the features of their ultrasonic vocalizations in response to the perceived environment.For example, bats change the duration and rate of echolocation calls as they approach prey 63,64 .Because bats show diverse and complex social behaviors, a vast repertoire of vocalizations (which may be learned depending on the species) and are well suited for laboratory research 65,66 , these animals have emerged as an ideal mammalian model for studying communication sound processing -not just a system for understanding echolocation.Recent work in bats has shed light on the neural mechanisms underlying the auditory processing of communication calls.In different bat species, neural selectivity for communication calls is present across brain regions; and population dynamics underlies call categorization in the auditory pathway (Inferior colliculus: [67][68][69] ; Auditory cortex: [70][71][72] ).This call selectivity is also present in affective processing areas such as the amygdala (Amg) and the PAG [73][74][75][76] .Current research investigates how the frontal cortex integrates auditory information to enable identity coding across individual bats interacting with conspecifics 77 .This discovery opened many questions regarding how social context modulates auditory processing.This nascent field of auditory communication processing in bats is occurring at an ideal time, as molecular tools that had traditionally only been available for common model species (i.e., mice and flies) are now available for a wider variety of species, including bats.In particular, the Bat1K project, a consortium that aims to generate chromosome-level genomes for all bat species, sequenced the genomes from 21 species 78 .This effort has enabled the first transgenic bat, allowing researchers to manipulate the expression of FoxP2 79 .In addition to target genes like FOXP2, other key molecules involved in the modulation of communication call signal processing are hormones and neurotransmitters.Dopamine, norepinephrine, serotonin, corticosterone, and adrenocorticotropic hormone release in the Amg increases in Cynopterus brachyotis bats (a bat species that does not use echolocation) when they produce or hear multi-harmonic distress calls 80 .Further, the distress calls in Cynopterus brachyotis elevate the levels of different proteins (TH, Nurr-1, DAT, D1DR) in the Amg of both the emitter and receiver engaged in live interactions, but not in bats listening passively to playback of modified distress calls 81 .These studies open the field to molecular approaches in bats that can lead the way for future studies across species.
Lastly, some bat species are vocal learners, a rare trait in the animal kingdom and valuable for studying the neural mechanisms of auditory communication.Young Saccopteryx bilineata bats learn their communication calls and practice their vocalizations similar to human infants during the babbling phase 82,83 .Rousettus aegyptiacus learn their vocalizations from their colony mates and can modify the acoustic parameters of their calls 84,85 .Phyllostomus discolor bats also learn their social vocalizations 86 , and modify acoustic parameters based on playbacks of communication calls 87 .
These studies pave the way for solidifying bats as mammalian models to study the neural mechanisms of auditory communication.Their expansive vocalization repertoires, complex social behaviors, similarities in brain structures with other mammals, and adaptability to laboratory life make bats strong candidates in the pursuit of comparative models in the field of auditory communication.

Looking to the future of comparative approaches
The field of auditory communication has seen significant advances fueled by the birdsong system.Nowadays, the use of mammals to study social communication is expanding, and collaborations among researchers using different models can help answer fundamental questions about the parity of systems across taxonomic groups.Specifically, employing the same approaches for a one-to-one comparison can provide insights and open new avenues for the field (Figure 1).
One potential example utilizing a comparative approach is to study the circuitry involved in auditory communication across species.Specifically, we emphasize the need and benefits of comparing the conserved neural networks underlying vocal production and audio-vocal integration, focusing on the PAG.In mammals, the PAG receives input from the POA and the Amg and projects to laryngeal and expiratory motor neurons 88 ; thus, this neural structure is ideally positioned to play a crucial role in modulating the vocal emission of social calls (Figure 2).Microstimulation experiments show that the PAG is involved in pathways that control the production of bat communication vocalizations 76 .Valentine et al. 89 extended this finding and showed that distinct regions of the PAG appear to be dedicated to producing bat communication sounds.
Direct simulations of specific PAG neurons elicited USV production in male and female mice 58 .Michael et al. 90 found that activating PAG-projecting neurons in the POA and central-medial boundary zone of the Amg stimulated and suppressed mouse USV production, respectively.In both species, direct evidence exists supporting that PAG is essential for the emission of social calls.One could then leverage the fact that bats also emit echolocation calls while mice do not echolocate.This difference opens the possibility of examining parallel circuitry that might underlie different modes of vocal emission and answer questions that could elucidate separate neural architectures for echolocation.The potential is intriguing, as some research supports the idea that parallel pathways are in place to produce echolocation and communication calls in bats.For instance, stimulation of the paralemniscal area (PLA), situated around the nuclei of the lateral lemniscus in the ventral midbrain, only elicits echolocation calls 76 .However, the production of communication calls through microstimulation of the PAG is not affected by PLA inhibition 91 .Like bats, mice also have a PLA, but the functional relevance to vocal emission is unknown.These similarities and differences in vocal emission and neural circuitry beg for further investigation into the connectivity, neuronal type, and circuit layout in bats compared to other animals allowing us to explore the potential conserved systems in producing and processing communication calls.

Concluding remarks
The advent of technologies enabling the exploration of questions in neuroscience across taxa has exciting implications for the field.Molecular, cellular, and behavioral techniques provide the toolkit to dissect the mechanisms by which animals produce and process communication.Bats, mice, and other mammalian models can build on the extensive research done by the bird song community, thus improving our understanding of the neural architecture underlying this complex system.Similarities across species will enable us to generalize and seek specific targets for research in other models, such as humans.For example, studying genes identified as potential targets in humans with communication disorders and dissecting their role in acoustic communication in parallel across mammalian species may reveal therapeutic candidates to improve patients' daily lives.Also, identifying the differences across taxa in how diverse species process communication sounds can provide insights into convergent evolutionary traits that are supported by different neural scaffolding.Furthermore, from an environmental perspective, knowing more about how different animals communicate and carry out their social interactions will provide vital information for future conservation efforts.Lastly, merging the fields of systems neuroscience and neuroethology promises a mutually beneficial interaction where auditory communication can be explored in diverse animal models especially suited for the questions at hand, while employing state-of-the-art and well-established techniques.

Grant information:
This work was supported by the National Institute on Deafness and Other Communication Disorders (R00 DC019145) and the National Institute of Mental Health (R01MH122752).
The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Amendments from Version 1
In this revised version of the manuscript, we addressed the reviewers comments as detailed in the response to reviewer 1.Briefly, we have changed the word species for strains when speaking of different mus musculus lab mice.We also clarified that the technological advances we refer to are those that enable sound source separation and localization.We've also reworded the sentence referring to the work on a point mutation FOXP2 in mice to more accurately describe the work of Groszer et al.We've clarified the location of the parameniscal (PLA) area and elaborated the figure legend of Figure 1 to enumerate the sophisticated techniques that are advancing comparative studies.
Any further responses from the reviewers can be found at the end of the article We define sophisticated techniques as chemogenetics, intersectional genetics, single-cell resolution connectomics, spatial transcriptomics, and computational approaches that will help bridge system neuroscience and neuroethology.

Figure 1 .
Figure 1.Graphical abstract of the proposed advantages of comparative work in the field of neural circuits mediating mammalian social communication.

Figure 2 .
Figure 2. Diagram showing proposed circuitry underlying vocal production in mammals.